Boreholes, which are also commonly referred to as “wellbores” and “drill holes,” are created for a variety of purposes, including exploratory drilling for locating underground deposits of different natural resources, mining operations for extracting such deposits, and construction projects for installing underground utilities. A common misconception is that all boreholes are vertically aligned with the drilling rig; however, many applications require the drilling of boreholes with vertically deviated and horizontal geometries. A well-known technique employed for drilling horizontal, vertically deviated, and other complex boreholes is directional drilling. Directional drilling is generally typified as a process of boring a hole which is characterized in that at least a portion of the course of the bore hole in the earth is in a direction other than strictly vertical—i.e., the axes make an angle with a vertical plane (known as “vertical deviation”), and are directed in an azimuth plane.
Conventional directional boring techniques traditionally operate from a boring device that pushes or steers a series of connected drill pipes with a directable drill bit at the distal end thereof to achieve the complex borehole geometry. In the exploration and recovery of subsurface hydrocarbon deposits, such as petroleum and natural gas, the directional borehole is typically drilled with a rotatable drill bit that is attached to one end of a bottom hole assembly or “BHA.” A steerable BHA can include, for example, a positive displacement motor (PDM) or “mud motor,” drill collars, reamers, shocks, and underreaming tools to enlarge the wellbore. A stabilizer may be attached to the BHA to control the bending of the BHA to direct the bit in the desired direction (inclination and azimuth). The BHA, in turn, is attached to the bottom of a tubing assembly, often comprising jointed pipe or relatively flexible “spoolable” tubing, also known as “coiled tubing.” This directional drilling system—i.e., the operatively interconnected tubing, drill bit and BHA, can be referred to as a “drill string.” When jointed pipe is utilized in the drill string, the drill bit can be rotated by rotating the jointed pipe from the surface, through the operation of the mud motor contained in the BHA, or both. In contrast, drill strings which employ coiled tubing generally rotate the drill bit via the mud motor in the BHA.
Irrespective of the well profile, whether it be horizontal, deviated, vertical, or any logical combination thereof, the wellbore trajectory must be mapped as precisely as possible to optimize harvesting of the hydrocarbon deposit. Historically, the path of a wellbore, or its “trajectory,” is determined by collecting a series of direction and inclination (“D&I”) measurements, such as inclination and azimuth, at discrete locations (“survey points”) along the wellbore path. From these angular measurements, in conjunction with the known length of the drill string, a theoretical model of the wellbore trajectory can be constructed. Azimuth and inclination may be measured by survey sensors positioned along the drill string. These measurements can be affected by inadvertent changes in the drill string or drilling environment. For example, the part of the string to which the sensors are attached may bend or “sag,” which can cause the borehole centerline to not necessarily point in the same direction as the centerline of the tool with the sensors.
Current practices in the drilling industry is to determine borehole position curvature by calculating the curvature between survey points (stations) as measured by a down hole survey instrument. The method most commonly used to define a well trajectory is called the Minimum Curvature Method, which is described, for example, by S. J. Sawaryn and J. L. Thorogood, in “A Compendium of Directional Calculations Based on the Minimum Curvature Method,” SPE Annual Technical Conference and Exhibition, Denver, Colo., 5-8 Oct. (2003), which is incorporated herein by reference in its entirety. Using this methodology, the wellbore trajectory is represented by a series of tangent vectors that are connected by a circular arc. Collections of other points, lines and planes can be used to represent features, such as adjacent wells, lease lines, geological targets, and faults. The relationships between these objects have simple geometrical interpretations, making them amenable to mathematical treatment.
An accurate borehole position is important in determining the separation from other wells, the delineation of oil and gas fields, and calculation of the volumes of petroleum in a reservoir. During an actual drilling operation, the path taken by the drilling tools is not along a single constant curve but rather consists of a series of curves of varying degree. Variations in the wellbore trajectory between the survey points are not taken into consideration in the Minimum Curvature Method when calculating the wellbore position. As such, the current methods commonly used to define a well trajectory do not provide the most accurate borehole position and curvature. In addition, the misalignment of the drilling tools within the complex borehole shape is not taken into account when correcting misalignment of the measurements taken at the survey stations. Current practices typically correct for borehole misalignment based on minimum curvature borehole shape. Such practices are unsatisfactory to offset borehole misalignment.
There is therefore a need to better determine the path of the wellbore between the survey stations and too more accurately calculate the wellbore position.